As is well known, safety in assuring contact effectiveness can be increased to a certain extent by the application of a number of mechanical measures. A frequently used arrangement for increasing switching safety resides, for example, in the arrangement of double contact springs or, very generally, in the arrangement of contacts in parallel connection. A prerequisite for reducing switching errors is provided by hunting contacts, however, this measure remains ineffective if the contact and contact surfaces always touch each other on the same surface region.
As a result of environmental influences, there are formed, for example, contaminated areas which, over time, have the effect of an insulating layer which will lead to switching errors. This error effect occurs in counter mechanisms which can be interrogated in the range of the upper counting decades whose settings are relatively rarely changed because generally low counting amounts occur. To counteract such disadvantageous phenomena, jumping contact springs are sometimes used. In using contact springs, fatigue phenomena in the resilient part of the contact arm already become apparent after relatively short periods of operation, so that the remaining contact spring pressure is no longer sufficient to achieve a satisfactory making of contact. A general increase of the contact pressure also leads only to a temporary improvement of the conductive connection and, on the other hand, leads to an increased load with respect to the drive torque.
Additional measures for improving the making of contact include the use of high-grade contact material which, of course, leads to a substantial increase in costs, especially when it is a multidigit interrogating device. It is in most cases not possible to simultaneously take into consideration all the measures mentioned thus far because of the questions of costs and space. In mechanical interrogating counters with low switching frequency, all of the above-mentioned measures have the disadvantage that sliding contacts of the higher decades with lower switching frequency are not kept ready to operate by means of self-wiping.